fix: correct tojson|safe usage in templates and update validator

- Remove |safe from |tojson in HTML attributes (x-data) - quotes must
  become " for browsers to parse correctly
- Update LANG-002 and LANG-003 architecture rules to document correct
  |tojson usage patterns:
  - HTML attributes: |tojson (no |safe)
  - Script blocks: |tojson|safe
- Fix validator to warn when |tojson|safe is used in x-data (breaks
  HTML attribute parsing)
- Improve code quality across services, APIs, and tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/letzshop_export_service.py

@@ -4,10 +4,10 @@ Service for exporting products to Letzshop CSV format.
 
 Generates Google Shopping compatible CSV files for Letzshop marketplace.
 """
+
 import csv
 import io
 import logging
-from typing import BinaryIO
 
 from sqlalchemy.orm import Session, joinedload
 
@@ -140,7 +140,9 @@ class LetzshopExportService:
         )
 
         if marketplace:
-            query = query.filter(MarketplaceProduct.marketplace.ilike(f"%{marketplace}%"))
+            query = query.filter(
+                MarketplaceProduct.marketplace.ilike(f"%{marketplace}%")
+            )
 
         if limit:
             query = query.limit(limit)
@@ -193,7 +195,9 @@ class LetzshopExportService:
     def _product_to_row(self, product: Product, language: str) -> dict:
         """Convert a Product (with MarketplaceProduct) to a CSV row."""
         mp = product.marketplace_product
-        return self._marketplace_product_to_row(mp, language, vendor_sku=product.vendor_sku)
+        return self._marketplace_product_to_row(
+            mp, language, vendor_sku=product.vendor_sku
+        )
 
     def _marketplace_product_to_row(
         self,